1. Field of the Invention (Technical Field)
The present invention relates generally to heat exchangers for liquid cooling of internal combustion engines, particularly heat exchangers with increased efficiency by local increased coolant velocity.
2. Background Art
It is known in the general art of internal combustion engines to provide some system for exhaust gas recirculation (EGR). EGR involves the return to the engine's intake manifold of some portion of the engine exhaust. Exhaust gases are diverted from the exhaust manifold through a duct or conduit for delivery to the intake manifold, thereby allowing exhaust to be introduced to the combustion cycle, so that oxygen content is reduced, which in turn reduces the high combustion temperature that contributes to excessive NOx formation.
The EGR method of reducing exhaust emissions has drawbacks. A specific problem is that EGR is most effective when the gases are cooled, which problem can be solved in part by using heat exchangers. It is known to provide heat exchangers in conjunction with EGR systems, whereby the heated exhaust passes through a heat exchanger core, together with a suitable coolant separated from the exhaust by a wall or other means. Such coolers may be “multi-pass”, in that either heated exhaust or coolant, or both, pass two or more times through the heat exchanger core. Exhaust gas enters a cooler at very high temperature and exits at much lower temperature.
Commercial diesel vehicles typically have significant cooling loads for heat exchangers employed in engine cooling, EGR systems and other applications. Prior art liquid cooled heat exchangers employing high temperature hot fluid, such as exhaust gas recirculated for emissions control, frequently result in boiling of the liquid coolant at low coolant flows. This phenomenon often results not from the bulk coolant temperature being too high but rather because the heat exchanger surface temperature exceeds the saturation temperature. The difference between the surface temperature and the liquid temperature, if high enough, can cause localized destructive film boiling to occur. The localized film boiling typically occurs in the gas inlet portion of the heat exchanger, where the temperature of the exhaust gas is highest. Coolant overheating and boiling can result in cracks and leaks in the heat exchanger, as well as performance degradation.
It is therefore desirable to provide a heat exchanger with variable coolant velocity at desired points to accommodate varying surface temperature issues. In particular, it is desirable to provide a heat exchanger with an increased coolant velocity proximate the gas inlet portion of the heat exchanger.
Against the foregoing background, the present invention was developed. The scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.